A film for optical application is commercialized later than a general polymer film that is used for packing materials, personal and household care products, vehicle, etc. However, as LCD-related technologies are developed and a study on a high functional film is advanced, the availability and demand of the film for optical application is being increased every day.
The film for optical film may include a viewing angle expanding film, an anti-reflective film, a compensation film, a luminous enhancement film, etc. A flim mainly used for the films for optical application is a polyester film.
The polyester film has excellent stability of physical property over a wide temperature range from a low temperature to a high temperature, excellent chemical resistance as compared to other polymer resins, good mechanical strength, good surface property, good uniformity of thickness, and excellent adaptability to various applications or process conditions, such that it may be applied to a condenser, a photographic film, a label, a pressure-sensitive tape, an ornamental laminate, a transfer tape, a polarizer, and a ceramic sheet, etc. Further, the demand of the polyester film is getting increased every day in line with the recently high-speed and automated trend.
The polyester film used in a field of a display has been used for a base film for a touch panel that is subjected to processes, such as hard coating processing through an offline coating, a film used for a PDP film, a base film used for a diffusion sheet included in a backlight unit, a prism lens sheet, a prism protective film, etc. a base film for an anti-reflective coating so as to prevent glare caused by light from the outside, etc.
The base film used in the field of the display has required several characteristics, such as processing and driving stability during processing, transparency, scratch resistant property, flatness, and light transmission. The reason why many requirements are needed is that the base film used in the field of the display should meet optical characteristics.
When flatness, which is one of characteristics required for the base film, is poor, tension is non-uniform in the manufacturing process of the base film to cause the slipping phenomenon, such that the scratch defect, etc., occur on the surface of the film and the applying thickness is non-uniform in a post-processing coating process, such that a partial coating defect occurs, thereby degrading the value of products.
When the scratch occurs on the base film, since problems, such as the occurrence of a black spot, which is an electrical defect, due to the non-uniform coating of the transparent conductive film on the scratched portion, the non-uniform coating in the post-processing processes such as the hard coating, etc. may occur, the scratch resistant property is needed. Further, the scratch causes the optical defects, thereby having a bad effect on the quality and yield of the products.
These properties required for the base film are required to increase the luminance improvement, thermal stability, processing property of the film. The deterioration of transparency, scratch resistant property, flatness, and total light transmittance deteriorates the luminance, reliability, and yield. The deterioration of luminance requires a brighter light source to obtain a required light amount and in order to obtain the high brightness light source, the cost of materials is increased and the high power consumption is required. As a result, the deterioration of luminance acts as a fatal defect factor to the base film used in the field of the display.
Therefore, in order to improve luminance, researches on the base film have been progressed. JP Patent No. 2006-208993 discloses a biaxially stretching polyester film comprized of a base film and a coating layer that is a light diffusion layer including binder and particles, JP Patent No. 2006-163378 discloses a polyester film having fine bubbles inside the film and prepared by laminating a coating layer including light stabilizer and antioxidant, which is stacked on the surface of the film, and JP Patent No. 1994-059108 discloses a polyester film having ruggedness formed on a base film and a base film whose both sides are stacked as a layer including light diffuser.
Further, a demand for the post-processing workability, in particular, the adhesive property with resin used for the post-processing has increased together with the requirements of the base film. When the adhesive property of the base film is low, in the post-processing process, the adhesive property between the base film and the post-processing resin is poor, such that the quality of products may be degraded.
In order to improve the adhesive property of the base film, JP-A-Hei 2-172143, JP-A-Hei 2-310048, and JP-A-Hei 3-67626 used a modified polyester resin based on graft modification as a coating resin, and JP A Hei 6-39154, and the like used a modified polyester resin together with a crosslinker. However, in performing different processing on both sides of the biaxially stretching polyester film, there is a problem in that one side has insufficient adhesive property.